A model of the compressor manifold system is used to illustrate some of the "classical" vibration mode shapes typically encountered with compressor manifold systems. While every system may not exhibit every mode shown, these are the typical modes of concern. Therefore, it is useful to understand these modes and the effective means of controlling them when designing reciprocating compressor installations.
Low Mode
An animation can be viewed here: Low Mode
It is called the low mode because it is often (but not always) the lowest frequency mode. The mode shape is basically rigid body motion of the cylinders and bottles all in-phase. If the frequency of this mode is coincident with a pulsation induced shaking force peak frequency or a mechanical excitation at a significant harmonic of compressor speed (generally 1x or 2x), high vibration levels and fatigue failures can result. The failure mode is often a torsional failure of the nozzles.
Angular Mode
An animation can be viewed here: Angular Mode
The angular mode involves horizontal motion of the suction and/or discharge bottles pivoting about the midpoint of their length. If this mode were excited, the highest stress would occur on the front or back of the nozzles near the cylinder flange.
Rotary Mode
An animation can be viewed here: Rotary Mode
The rotary mode involves the suction and discharge bottles moving out-of-phase, but basically in-plane of the manifold system. If this mode were excited, the highest stress points would be on the sides of the nozzles at the bottle-nozzle junction.
Cylinder Resonance Mode
An animation can be viewed here: Cylinder Mode
The cylinder resonance mode involves out-of-phase motion of the cylinders. With more than two cylinders, various combinations of one cylinder moving out-of-phase with the others will occur. Therefore, there will be several cylinder resonance modes. The highest stress levels occur on the sides of the nozzles.
